So we think we know the problem; what's the proposed solution?

I came up with a stupid idea that maybe could be toyed with, if for no other reason than to see how it works in reality: You can only see and interact with accounts from people who registered at the same time you did, within a year, perhaps. If you registered three years ago, you can't see anything from people who registered two years or one year ago, though they can see you if they want.

This gets rid of reposts and "newbies", because you hang out with a crowd that has seen what you've probably seen and you never have to deal with the rerun syndrome. Do you really need to interact with all 100,000,000 active users of a site? Even the 1,000 active users of a small site? See all the stuff they post? If you really want to see the stuff newbies make, then you can have the option to see all posts from all users and so on. Or maybe if it gets "upvoted" or starred enough you get to see it despite your settings (though you can disable that if you want) so you don't miss out on a newbie who happens to know what he's talking about because he's an expert in the field.

Just a thought, perhaps a completely groundless and unexamined thought on a subject I barely understand, but it is what it is. This post I thought of has to go somewhere, so it may as well be here.